Alternating current control device



Feb. 24, 1959 J, WALSH 2,875,378

ALTERNATING CURRENT CONTROL DEVICE FiledMay 21, 1954 2 Sheets-Sheet 1 IFig IN VEN TOR.

Feb. 24, 1959 P. J. WALSH ALTERNATING CURRENT CONTROL DEVICE 2 Sheets-Sheet 2 Filed May 21, 1954 Fig 3 Fig 4 United States Patent'O 2,875,378ALTERNATING CURRENT CONTROL DEVICE Philip J. Walsh, San Francisco,Calif.

Application May 21, 1954, Serial No. 431,449

. 5 Claims Cl. 315-100 This invention relates to the control of currentthrough a load having a negative resistance, or run-away,characteristic. i P

The fluorescent lamp is such a load and requires auxiliary equipment todevelop a voltage across the lamp that is high enough to start the arcin the lamp and to control the current flow through the lamp, since theresistance of the lamp decreases as current flows.

Heretofore, this has been accomplished by employing an auto transformerto raise the line voltage high enough to start the arc and controllingthe current flow by means of a very large ironcore choke coil in serieswith the lamp. l

Special high 'reactance transformers are used to supply the high'voltage and current control function through long lengths of luminoustubing. The open circuit output voltage of these transformers can be ofthe order of 15,000 volts and when the proper luminous tube load isconnected across the output, this voltage valuefalls to less than 7,000voltsafter the arc starts in the tube. With this type of transformerthis high voltage loss is necessary in order to control the currentthrough the tube. The powerfac'tor of such a transformer is lagging andis of the order of 50%. The light output of tubing connected to such atransformer varies with variations inline voltage. f

, It is one of the objects of my invention to obviate thesedisadvantages and to provide a low cost, light weight arid vefiicientcontrol device that operates a given length of luminous tubing at lessthanhalf the required output voltage of the conventional transformer.

It is another object of m invention to provide a control device forelectric discharge tubes having a high power factor; p H p p H y .J

i It isflstill another object of myinvention to provide a control devicethat produces a constant light output from the lamp regardless of largeline voltage variations.

My invention possesses many other advantages and has other objects whichmay be made more easily apparent from a consideration of one embodimentof my invention. For this purpose, I have shown a form in the drawingaccompanying and forming part of the present specification.

I shall now proceed to describe this form in detail, which illustratesthe general principles of my invention; but it is to be understood thatthis detailed description is not to be taken in a limiting sense, sincethe scope of my invention is best defined by the appended claims.

Referring to the drawings:

Figure 1 is a system diagram depicting one form of my invention;

Figures 2, 3, and 4 are drawings for facilitating explanation of theinvention.

In Fig. 1 I show a fluorescent-tube lighting circuit employing apreferred embodiment of my invention. As is illustrated in thisschematic diagram, a generator 2, which can be the ordinary commercialpower supply, is connected to a three-legged transformer having thecustomary laminated steel core. The core comprises an outer leg 4,carrying a primary winding 6, a centerleg 8, on which is wound asecondary winding 10, and another outer leg 12 carrying a second primarywinding 14. Windings 6 and 14 are connected in series with the source 2,the connections being such that the magnetomotive forces setup bycurrent through the winding develop fluxes in the legs and in yokes 16and 18 respectively, joining the three legs, in the directions indicatedby the solid arrows. The secondary winding 10 connects in series withthe load, a series condenser 20 preferably, but not necessarily, beinginterposed in the circuit in series with the fluorescent lamp 22.

The parts are so proportioned that when the loadis disconnected thevoltage drop across the winding 14 is less than that across the winding6. This can be accomplished ineither of two ways; the cross-section ofthe leg 12 may be smaller than that of legs 4 and 8, the number of turnsin coil 14 may be less than the turns of coil 6, or the two expedientsmay be combined. As a result of this arrangemenn at instants when highervalues of cur-- rent are being drawn by the load and, as a result,rela-- tively high counter-magnetomotive forces are set up by the coil10 in the core, the flux distribution through the core will be asindicated by the dotted arrows.

In a test of my invention satisfactory operation was; obtained when thenumber of turns in the coil 6 was. greater than the number of turns incoil 14.. That is, inq. the test referred to, coil 6 contained 800 turnsand coil;.

14 contained 600 turns.

Several models embodying the principles of my irivention wereconstructed and tested. As shown in the draw ing, the core is arrangedso that the leg 12 is of one half the-- cross-sectional area of the legs4 and 8 and the yokes 16 and 18. This is the most economical form,however, other core forms can be employed provided the turns in thecoils are properly proportioned to produce the necessary Operatingmagnetic flux characteristics.

The core can be made of low costsilicon steel lamina tions, preferablythe sheets marketed as Transformer No., The laminations are 52, byseveral steel companies. tightly stacked and lapped at the joints inorder to avoid! air-gaps. The yokes 16 and 18 are short, sothat the:coils are as close together as possible.

I When the flu x linking the winding 10 is changing, an

electromotive force equal to the time rate of change of the flux linkageis generated in this winding and tends to send current through winding10 ina direction to op pose the change in flux; The luminous tube load22 connected across the terminals of coil 10 acts as a switching loadwith a negative resistance characteristic. That is, during each halfcycle, the arc starts in the tube when the voltage across the tubereaches a definite value and stops when the voltage value is too low tomaintain the This action develops a sort of rhythm between the load 22and the flux in the leg 8. The flux in the leg 12 reaches saturationvalue before the current flowing in coil 14 reaches its maximum value.When this point in the current cycle is reached, only the coil 6continues to produce a flux change in the leg 8 as the current builds upto its maximum value.

The control of the current is due to several factors which combine toproduce a constant current effect. The load current flowing in coil 10develops an opposing magnetomotive force in leg 8 thus forcing some ofthe flux from leg 4 over to leg 12. This flux develops a bucking'voltage in coil 14 at one instant and in coil 6 at another instantwhich acts to maintain the value of the current substantially constantregardless of very large line voltage variations. Figure 2 is anoscillogram of the voltage wave across the terminals of coil 6 andFigure 3 is an oscillogram of the voltage wave across the terminals ofcoil 14, which shows this eflect. These oscillograms were obtained byconnecting an oscilloscope directly across the coils'and indicate thatthe peak voltage across coil 6 equals that across coil 14 and has avalue of 140 peak volts when the applied voltage of source 2 has aneffective value of 110 volts. Figure 4 is an oscillogram of the voltageacross the terminals of the source 2. It is seen that when this voltagereaches its maximum value, the voltage across coil 6, as shown in Figure2, is passing through its maximumyalue andFigure 3 shows that thevoltage across the coil 14, is very near zero value at this instant.

In one comparative test of my invention, the load 22 comprised afourteen foot length of twelve millimeter tubing, According to thestandard chart published by the manufacturers of luminous tubetransformers, this length of tubing requires a conventional transformerhaving an open circuit output voltage of 5,000 volts. The open circuitvoltage developed across the coil 10 during this test was 2,200 volts.The light output of the tube, as measured by a foot-candle meter, wasequal to that developed by the tube when connected to the conventionaltransformer. The condenser 20 had a value of one tenth microfarad andraised the power factor from 79%, without the condenser connected in thecircuit, to 92% with the condenser connected as shown.

In another test of my invention the load 22 comprised an 8 foot coldcathode lamp having a diameter of one inch and operating current of 120milliamperes. Condenser 20 had a value of six tenths microfarad. Thelight output from the tube was substantially constant while the linevoltage was varied and the power factor was 95%.

I claim: 1

1. A transformer for supplying gas-discharge tubes comprising a closedmagnetic core having two end legs and a center leg connected by yokes, apair of primary windings disposed on the two end legs of said core andconnected in series so as to develop magnetomotive forces acting in thesame direction through the center leg of said core, saidseries-connected windings being adapted for connection'to a. relativelylow-voltage source of alternating current power and a relativelyhigh-voltage secondary winding disposed on said center leg, the productof the number of primary turns on one of said end legs times thecross-sectional area of the leg whereon it is disposed being materiallygreater than the product of the number of turns of the other of said endlegs times the cross-sectional area of the leg whereon it is disposed.

2. A transformer for supplying gas-discharge lighting tubes comprising aclosed magnetic core having three legs connected by yokes, said legsincluding a first end leg and a center leg of substantially equalcross-section and a second end leg of materially smaller cross-section,a section of primary winding disposed on said second end leg, a secondprimary winding section having a number of turns at least equal innumber to those of said first primary winding section disposed on saidfirst end leg and so connected in series with said first primary windingsection as to develop magnetomotive 'forces in the same directionthrough said center leg, said primary winding sections being adapted forconnection to a relatively low voltage source of alternating currentpower, and a relatively high voltage secondary winding disposed on saidcenter leg.

3. A transformer as defined in claim 2 wherein the number of turns ofsaid second primary winding-sections is materially greater than thenumber of turns of said first primary winding sections.

4. A transformer for supplying gas-discharge tubes comprising a closedmagnetic core having two end legs and a center leg connected by yokes,primary winding-seetions adapted for connection to a relatively lowvoltage source of alternating current power disposed on said end legsand so connected in series as to develop magnetomotive forces acting inthe same direction across said center leg, one of said primary windingsections having a materially less number of turns than the other, and arelatively high voltage secondary winding disposed on said center leg.

5'. In a gas-discharge tube lighting circuit, a transformer comprising aclosed magnetic core having three legs connected by yokes and includingtwo end legs and a centerleg, aprimary circuit adapted for connection toa power source and comprising two winding'sections connected in seriesand disposed on said two end legs respections times the cross-sectionalarea of the core-leg where! tively, the product of the number of turnsof one of said winding sections times the cross-sectional area of thecore on which it is disposed being materially greater than the productof the number of turns of the other of said secon it is disposed and asecondary circuit consisting essentially of a secondary winding disposedon the. center leg of said core, gas discharge apparatus having anegative impedance characteristic and a condenser connected in series.

References Cited in the file of this patent UNITED STATES PATENTS 1616,286 'Great Britain Jan '19, 1949

